Method of forming magnetite thin film

ABSTRACT

A magnetite film is produced by depositing a thin film of pure iron on a substrate, oxidizing the film, depositing a second film of iron on the iron oxide film, and annealing the composite film at a temperature below about 560*C and preferably from 350*C to 400*C and then stripping excess iron from the film.

United States Patent Nicolet et al.

[ Jan. 14, 1975 METHOD OF FORMING MAGNETITE THIN FILM Inventors: Marc-Aurele Nicolet, Pasadena,

Calif.; Christopher H. Bajorek, Katonah, N.Y.; Joseph Shao-Ying Feng, Kenilworth, Ill.

California Institute of Technology, Pasadena, Calif.

Filed: May 5, 1972 Appl. No.: 250,565

Assignee:

US. Cl 117/237, 117/235, 117/239, 117/240 Int. Cl. H011 10/00 Field of Search 117/236-240, 117/235 References Cited UNITED STATES PATENTS 11/1963 Bean 117/240 X 3,148,079 9/1964 Banks et al. 117/237 3,328,195 6/1967 M ll7/l2l X 3,620,841 11/1971 Comstock et al 117/237 3,652,334 3/1972 Abeck et a1. 117/235 X 3,703,411 ll/l972 Melezoglu 117/235 X Primary ExaminerWilliam D. Martin Assistant Examiner-Bernard D. Pianalto Attorney, Agent, or FirmLindenburg, Freilich, Wasserman, Rosen and Fernandez 10 Claims, 5 Drawing Figures PATENTED JAN 1 M 1 1 qI-l IRON 5O LJEQE METHOD OF FORMING MAGNETITE THIN FILM ORIGIN OF THE INVENTION The invention described herein was made with partial support of the National Science Foundation, an agency of the United States Government.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to production of ferrite magnetic films and, more particularly, to the equilibrium induced formation of thin films of magnetite at low temperature.

2. Description of the Prior Art The current interest in ferrite thin films arises from its applications to computer memories, microwave integrated circuits, and magneto-accoustical devices. Over the past 14 years, several methods for producing ferrite thin films have been devised, meeting with varying degrees of success and varying qualities of films. They include pyrolytic spraying of metal-organic compounds, oxidizing metal films in appropriate atmospheres and at appropriate temperatures, depositing an appropriate metal hydroxide or halide and reacting the deposited film with other chemicals on a hot substrate, sputtering, spraying, grinding, or sintering ferrites onto substrates, or reducing hematite in a water hydrogen atmosphere.

Many of these processes involve high temperature and require special substrates. Several of these processes involve the use of dangerous chemicals and hydrogen as the annealing gas atmosphere which requires special handling and equipment to avoid injury to personnel. Furthermore, the processes include extraneous atomic species not necessary in the final product.

OBJECTS AND SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a process for producing thin films of magnetite at low temperature.

A further object of the invention is the provision of a process for producing magnetite thin films which involves only those elements desired in the final product, that is, iron and oxygen.

Yet another object of the invention is the provision of a process of producing thin films of magnetite that is compatible with current thin film technology as well as monolithic integrated circuit technology.

These and other objects and many attendant advantages of the invention will become apparent as the description proceeds.

In accordance with the invention, a low temperature process for producing magnetite thin films comprises the steps of depositing a thin film of iron on a substrate, oxidizing the film into hematite (a-Fe O at a temperature below about 560C, depositing another film of pure iron on the oxidized film and annealing the film at a temperature in the range of 350C to 400C and then removing excess pure iron from the surface of the magnetite film without significantly affecting the magnetite film. The phase transition from hematite to magnetite is demonstrated by change in color of the oxide from bright, red-orange to a dark color apparently black with overtones of green and brown. The films are unambiguously defined as magnetite by abackscattering spectrography, X-ray powder diffractometry and observations of electrical, magnetic and optical properties.

The invention will now become better understood by reference to the following detailed description when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of a substrate with an iron film;

FIG. 2 is a schematic view illustrating convertion of the iron film to hematite;

FIG. 3 illustrates the hematite film overcoated with an iron film;

FIG. 4 illustrates the conversion of the hematite-iron film to magnetite; and

FIG. 5 illustrates the final magnetite product.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, in the first step of the process, a thin film 10 of pure iron is deposited onto a substrate 12. The film 10 may be deposited by various means such as by vacuum deposition, decomposition of an iron carbonyl or suitably by RF sputtering onto the substrate from a source 14 of high purity iron. The deposition is suitably conducted in a vacuum chamber 16 at a pressure typically about 10 Torr.

The thickness of the film may be varied over wide limits. The film thickness is at least sufficient to form the desired magnetite film. The thickness may exceed the final magnetite thickness and a layer of unconverted iron will remain between the substrate and the hematite film. The film thickness is suitably below microns and films less than 1 micron such as below 5,000 A in thickness demonstrate satisfactory physical, optical, electrical and magnetic properties. The substrate 12, is preferably an insulator capable of withstanding the temperatures and chemicals utilized in the processing and has a coefficient of expansion compatible with the deposited films. The substrate is suitably an inorganic material, such as refractory metal oxides, alumina or sapphire, or a silicon oxide, such as quartz or thermally oxidized Si, or any of several glasses.

Optionally, the substrate may be outgassed prior to deposition by heating the substrate in a vacuum better than 10 Torr to a temperature of about lOO-400C. Deposition of the pure iron film may be done at elevated substrate temperatures in the same general temperature range. A magnetic field may be applied parallel to the film during deposition to establish preferred orientation or anisotropy in the magnetization of the pure iron film.

The pure iron film is then oxidized by placing the iron film coated substrate in an oven 18, as shown in FIG. 2, introducing an oxygen containing heating gas such as air or oxygen into the oven and heating the film to a temperature below about 560C. The binary phase diagram of the oxygen iron system has only two compounds below 560C. In the presence of excess oxygen, hematite is the only stable compound.

A hematite film 20 or layer in the submicron range is formed in accordance with the invention by annealing the pure iron film 10 in the presence of excess oxygen at a temperature of from 450 to 550C. The annealing reaction is suitably conducted at atmospheric pressure. This forms a clear red oxide film since anti-ferromagnetic a-Fe O is the only stable iron oxide in an oxygen rich atmosphere at these temperatures. The temperature affects only the oxidation rate and not, the compound formed. The oxidation may proceed completely through the iron film to the substrate 12 or a layer of pure iron may remain under the hematite film.

The hematite coated substrate is then returned to the vacuum coating chamber 16 for deposition of a further layer 22 of pure iron onto the hematite layer as shown in FIG. 3. The amount of iron required for completeconversion to magnetite is only about 12 percent of the hematite layer on the basis or iron content. However, the thickness of this film is usually controlled to be over 50 percent of the thickness of the original film to provide excess iron that will protect the oxide film against possible undesired reactions with the ambient annealing atmosphere. Thin evaporated films have high internal stress and large effective surface area. They are, therefore, more reaction than bulk forms of the same materials and phase transitions can occur rapidly even at relatively low temperature.

The coated substrate is then returned to the annealing furnace l8 and annealed in an inert environment, typically under vacuum, preferably no less than 10* Torr and at a temperature below about 560C. at 350C to 400C. The red hematite film 20 is completely converted to a dark brown to green magnetite film 24 in a period of about less than 8 hours as shown in FIG. 4. The binary-phase equilibrium diagram establishes that the only stable iron oxide compound at these temperatures and conditions is magnetite.

Excess iron 26 is then removed from the surface of the magnetite film 24 by menas of an iron solvent that does not substantially affect the magnetite film. Suitably, the coated substrate is dipped into a bath of solvent, rinsed and then dried. The solvent is an aqueous solution of HNO having a normality below about 3 or a solution of one part bromine in five parts of methyl acetate by volume. The final product as shown in FIG. 5 is the substrate 12 coated with a thin film 24 of magnetite.

A specific example of practice follows:

EXAMPLE A film of pure iron about 1,000A. thick and one cm in diameter was deposited onto the fiat surface of a sapphire substrate by RF induction heating evaporation at a vacuum of better than Torr. The iron film was then annealed at 450C to 550C in an atmosphere containing excess ogyxen. A second layer of pure iron about 1,000A. was deposited onto the hematite fire. The iron overcoated hematite film was then annealed at 350C to 400 in an inert ambient, normally a vacuum of better than 10 Torr for about 8 hours. Excess iron was removed in a 1:5 by volume solution of bromine in methyl acete or a 10% by volume solution of HNO Optical microscopy with 300 X magnification, showed no evidence of pinholes in either the original iron or any of the resulting oxide films. The thickness of the original iron film determines that of the hematite film, which in turn controls the thickness of the final magnetite film. The thickness of the resulting magnetite film was estimated from bulk properties to be a factor of 2.34 greater than the initial iron film thickness.

Several tests indicate that the film formed is in fact magnetite. The transition from hematite to magnetite was verified by backscattering spectrography with 2.0 MeV He ions. The iron signal of a film of hematite covered with iron on a sapphire substrate taken before anneal was compared with that of another film taken after conversion. The data shows that after conversion the amount of iron contained in the oxide had increased at the expense of the pure iron film. The change in both the thickness and the iron content of the oxide is consistent with the transformation to magnetite. The average yields gave a backscattering yield of 0.948:l.00 before and after conversion. Assuming Braggs rule, the expected Fe O -,:Fe O yield ratio is 0.953: 1 .00.

The principal X-ray pattern diffraction lines for several iron oxide using Cu-Ka radiation are consistent with a polycrystalline cubic structure and a lattice parameter of 8.39 i 0.003A. The only phase consistent with this structure is magnetite. The one-to-one correspondence of the diffraction pattern peaks to Fe O lines excludes the possibility of significant amounts of wustite (FeO) or most phases of Fe O The lattice parameter of maghemite (y F e 0 is very close to that of magnetite, but this phase is eliminated by the increased iron content evident in the backscattering data. The half-width of the (31 1 peak) of less than 0.2 indicates that the microcrystals are relatively free of strains and defects.

Electrical properties of the films were measured in the four-point probe and van der Pauw configurations. At room temperature the resistivity is 8.5 X 10 ohmcm. This value is reproducible and the conduction is not due to any residual iron on the surface. While the literature indicates a value of about 7 X 10 ohm-cm for bulk resistivity, this is believed to be the closest reported value of resistivity in thin films with respect to the bulk values. The sign of the Hall voltage generated indicates that the carriers are electrons.-

The magnetic properties of some films were measured with a 60hz, lKOe peak field range hysteresis loop tracer, a 20 KOe field range force balance magnetometer and with a perpendicular ferromagnetic resonance (FMR) spectrometer in the 1 to 8 Ghz frequency range. The coercivity and remanance of films on glass substrates ranged from 220 to 280 Oe and 50 to 60 percent, respectively. The coercivity and remanance of films on silicon and oxidized silicon substrates ranged between 300 and 400 Oe and 45 to 55 percent, respectively.

The observed coercive fields are consistent with previously reported values. The films show no measureable planar uniaxial anistropy. The saturation magnetization of the films as determined for six films from FMR and force balance magnetometer measurements was 471' M 5.2 i- 03 k6 as compared to 6.0 kG for bulk magnetite. The agreement of the FMR and force balance magnetometer measurements is consistent with absence of any significant perpendicular anisotrpy. The gyromagnetic ratio as determined from FMR is 'y/21r 1.95 i 0.10 mhz/Oe. All FMR line widths were less than Oe, much sharper than the l,7001,500 0e values reported for bulk magnetite.

It is apparent that the present invention provides a low temperature process for providing thin films of magnetite in the submicron range in a process involving low temperatures and utilizing only iron and oxygen as the precursor atomic species. The processes are compatible with current thin film technology as well as monolithic integrated circuit technology and will find many applications.

It is to be understood that only preferred embodiments of the invention have been described and that numerous substitutions, alterations and modifications are all permissible without departing from the spirit and scope of the invention as defined in the following claims.

What is claimed is:

1. A method of forming a magnetite film comprising the steps of:

depositing a thin film of pure iron on a substrate;

annealing at least the upper portion of the deposited iron film to form a hematite film, by exposing the deposited iron film to oxygen at a temperature below about 560C;

depositing a layer of pure iron on the hematite film;

and

annealing the composite iron coated hematite film in an inert environment at a temperature below about 560C at a pressure not less than about Torr, thereby converting the composite film to a magnetite film.

2. A method according to claim 1 in which the iron content of the iron layer deposited on said hematite film is greater than 12 percent of the iron content of the hematite film, further including the step of removing excess iron from the surface of the magnetite film by applying an iron solvent to said surface and dissolving said excess iron.

3. A method according to claim 1 in which the converting step is conducted in the presence of excess oxygen and wherein the step of annealing is conducted in an inert environment free of oxygen or hydrogencontaining gases.

4. A method according to claim 3 in which the converting step is conducted at a temperature from 400C to 500C.

5. A method according to claim 1 in which the thickness of the magnetite film is below microns.

6. A method according to claim 1 further including the step of orienting the iron film by applying a magnetic field parallel to the iron film during deposition thereof.

7. A method according to claim 1 in which the substrate is an inorganic, refractory insulator having a coefficient of expansion compatible with that of said iron, hematite and magnetite films. V

8. A method according to claim 1 in which the iron content of the iron layer is at least 12% of the iron content of the hematite film and said annealing is conducted at a temperature of from 350C to 400C.

9. A method according to'claim l in which the iron content of the iron layer is at least 50 percent of the iron content of the hematite film and said annealing is conducted at a temperature of from 350C to 400C, further including the step of removing excess iron from the surface of the magnetite film by applying an iron solvent to said surface and dissolving said excess iron.

10. The method according to claim 1 in which the iron content of the irorr layer is at least 12 percent of the iron content of the hematite film.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3!860!450 'Dated 1975 Marc-Aubrele Nicolet, et al Inventor (s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3 line 20, "reaction" should be --reactiveline 35, "menas" should be --means- Column 4 line 17, "CuKa" should be---CuK-.--

line 59, after "with" insert the- Column 5 line 15 (Claim 1) delete "annealing" and insert --oonverting-.

Signed and sealed this 13th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks FORM PO-1050 (10-69) USCQMM-DC 60375-p69 "is. GOVERNMENT PRINTING OFFICE: I 

1. A METHOD OF FORMING A MAGNETITE FILM COMPRISING THE STEPS OF: DEPOSITING A THIN FILM OF PURE IRON ON A SUBSTRATE; ANNEALING AT LEAST THE UPPER PORTION OF THE DEPOSITED IRON FILM TO FORM A HEMATITE FILM, BY EXPOSING THE DEPOSITED IRON FILM TO OXYGEN AT A TEMPERATURE BELOW ABOUT 560*C; DEPOSITING A LAYER OF PURE IRON ON THE HEMATITE FILM; AND ANNEALING THE COMPOSITE IRON COATED HEMATITE FILM IN AN INERT ENVIRONMENT AT A TEMPERATURE BELOW ABOUT 560*C AT A PRESSURE NOT LESS THAN ABOUT 10**-7 TORR, THEREBY CONVERTING THE COMPOSITE FILM TO A MAGNETITE FILM.
 2. A method according to claim 1 in which the iron content of the iron layeR deposited on said hematite film is greater than 12 percent of the iron content of the hematite film, further including the step of removing excess iron from the surface of the magnetite film by applying an iron solvent to said surface and dissolving said excess iron.
 3. A method according to claim 1 in which the converting step is conducted in the presence of excess oxygen and wherein the step of annealing is conducted in an inert environment free of oxygen or hydrogen-containing gases.
 4. A method according to claim 3 in which the converting step is conducted at a temperature from 400*C to 500*C.
 5. A method according to claim 1 in which the thickness of the magnetite film is below 100 microns.
 6. A method according to claim 1 further including the step of orienting the iron film by applying a magnetic field parallel to the iron film during deposition thereof.
 7. A method according to claim 1 in which the substrate is an inorganic, refractory insulator having a coefficient of expansion compatible with that of said iron, hematite and magnetite films.
 8. A method according to claim 1 in which the iron content of the iron layer is at least 12% of the iron content of the hematite film and said annealing is conducted at a temperature of from 350*C to 400*C.
 9. A method according to claim 1 in which the iron content of the iron layer is at least 50 percent of the iron content of the hematite film and said annealing is conducted at a temperature of from 350*C to 400*C, further including the step of removing excess iron from the surface of the magnetite film by applying an iron solvent to said surface and dissolving said excess iron.
 10. The method according to claim 1 in which the iron content of the iron layer is at least 12 percent of the iron content of the hematite film. 